The demand for smaller thickness and lighter weight is not limited to the housings of mobile electronic devices such as portable computers and cell phones and it has recently become intensive in the field of general electronic devices. In particular, the chassis and the internal mechanistic parts of a copying machine and the like are required to have not only high dimensional precision and various kinds of strength associated with handling but also reduction in thickness and weight. As a result, injection moldings are needed that desirably have nonuniform sections, i.e., the portions that do not require strength are made thin and lightweight whereas the portions that require strength are made thick, and which still have good dimensional precision. In other words, it is required to meet both requirements for strength and lightweight by providing a design in which the portions that require strength are reinforced with thick ribs whereas the portions that do not require strength are made as thin as possible. Under these circumstances, a molding method is needed by which even the portions that are thin-walled and have long flow distances can be adequately filled with a resin during molding.
In order to ensure that even the portions that are thin-walled and have long flow distances can be adequately filled with a resin, one may enhance the flowability of the resin. In injection molding of thermoplastic resins, the flowability of a molten resin determines not only the ease in filling the mold cavity but also the probability that after filling the cavity, sufficient pressure is transmitted to its interior, particularly to the resin which forms the thin-walled portions at the end of resin flowing; hence, the flowability of a molten resin also affects the dimensional precision of moldings and is an important factor that determines the processability of resins.
One index of flowability is the viscosity of molten resin. Thermoplastic resins have high melt viscosity and are poor in flowability as molding materials. Hence, in the case of thin-walled parts, incomplete resin filling often occurs.
In order to lower the viscosity of molten resin and thereby improve the flowability, it is effective to increase the molding temperature; however, in the case of a resin for which the molding temperature is close to its decomposition temperature or a resin incorporating additives such as less heat-stable flame retardants, the resin itself or the additives may undergo thermal decomposition and problems are likely to occur as exemplified by the decrease in the strength of moldings, the formation of foreign matter due to the deteriorated resin, the staining of the mold and discoloration. Yet another problem is delayed cooling of the resin in the mold which contributes to prolonging the molding cycle time.
The following methods are conventionally known to be capable of improving the flowability of molten resin without increasing the molding temperature.
(1) Method of reducing the molecular weight of the resin by lowering its average molecular weight or broadening the molecular weight distribution, particularly by increasing the low-molecular weight component.
(2) Method of introducing a comonomer into the molecule.
(3) Method of adding a low-molecular weight oily substance such as mineral oil or a plasticizer such as a higher aliphatic acid ester.
(4) Method of dissolving carbon dioxide which acts as a plasticizer.
To further describe method (4), reference may be had to WO 98/52734 which teaches that carbon dioxide dissolved in a resin works as a plasticizer for the resin to lower its glass transition temperature.
It is known to produce foamed molded articles using molten resins having a gas such as carbon dioxide dissolved therein. For example, the specification of U.S. Pat. No. 5,334,356 discloses a method in which carbon dioxide used as a blowing agent is supplied into a molten resin as it flows part of the way through an extruder, thereby molding a fine and highly expanded microcellular foam. The official gazettes of Japanese Patent Publication No. 57213/1988 and Japanese Patent Laid-Open No. 34130/1999 have descriptions of an injection molding machine for foam molding using a gas and according to their teachings, a blowing gas is supplied at a site which is part of the way through an extruder and a mechanism for preventing resin back-flow of a is provided in both the first and second stages of the screw and, optionally, also at the gas supply valve.
However, method (1) mentioned above lowers impact strength and chemical resistance although it increases flowability; method (2) lowers hot rigidity; and method (3) has problems such as the plasticizer lowering hot rigidity or being deposited on the mold to stain it during molding. Method (4) has the advantage of not causing the problems encountered in methods (1)-(3), however, the desired improvement in flowability is difficult to achieve if an insufficient amount of carbon dioxide is dissolved in the molten resin.
The following two methods may be employed to dissolve gases such as carbon dioxide in the resin. In the first method, a resin in particulate or powder form is preliminarily placed in a carbon dioxide atmosphere and supplied into the molding machine after it has been allowed to absorb carbon dioxide. The amount of carbon dioxide absorption is determined by the pressure of carbon dioxide, the temperature of the carbon dioxide atmosphere and the time for which carbon dioxide is absorbed. In the other method, carbon dioxide is supplied and dissolved in the plasticized resin in the cylinder of the molding machine.
To supply a gas such as carbon dioxide into molten resin in the commonly used in-line screw type and screw preplunger type injection molding machines in which the screw rotates intermittently, the gas supply pump has to be operated in accordance with the amount of resin transfer which varies with time as the screw stops or starts rotating, so it is difficult to ensure that the amount of the gas dissolved in the resin is controlled at constant level.
The present invention has been accomplished in the light of the aforementioned conventional problems. Accordingly, an object of the present invention is to provide an injection molding method which comprises supplying a gas into a plasticizing cylinder and injecting a molten resin having the gas dissolved therein, and in which a quantitative and economically required volume of a gas such as carbon dioxide can be dissolved in the molten resin even if the screw rotates intermittently in the plasticizing cylinder.